How to get Echo Shells in Horizon Forbidden West

gamepur.com:

In Horizon Forbidden West, you will constantly be hunting and gathering supplies and resources to fuel your crafting and ammo needs. This is important since you do not want to waste all of your Metal Shards buying all the things you need when you can just craft them while on the go. Echo Shells are parts required for preparing tear and knockdown ammunition. Here is where you can find it.

If you are looking to replenish your Echo Shells so you can make a bunch of tear or knockdown ammo, you will want to hunt down any machines that use sonic or compressed attacks. Our recommended targets are Longlegs and Skydrifters. Each has a 100% chance to drop an Echol Shell when defeated, so you can constantly fast travel between two of their sites and kill them. There is a Longleg site to the southwest of Cauldron: MU and a Skydrifter site north of there. Both spots have nearby campfires that we recommend traveling back and forth to.

If you do not want to fight machines, there are merchants that you can buy Echo Shells from. The Salvage Contractor at Greenswell will trade them with you, or you can occasionally find Hunter Merchants that have them in stock. To save your Metal Shards, though, we recommend farming the two areas mentioned above to get them and some other resources.